A. Field of Invention
The present invention relates generally to the field of earthquake safety systems for manmade structures, and more particularly to a method and apparatus for mitigating load imposed upon a structural frame using one or more tensioned tendons arranged in space to provide optimal reaction to the imposed lateral loads.
B. Description of the Prior Art
Ever since mankind began building structures to live and work in, the destructive power of earthquakes has been a looming threat to life and limb, especially in certain geographical regions, with the potential to flatten entire cities and cause thousands of deaths in a matter of seconds. In China, for example, extreme devastation occurred in the year 1556 when an earthquake is reported to have killed 830,000 people. Even in recent times, the death toll in China from earthquakes has been enormous. From 1920 to 1976, China has seen nearly 800,000 deaths from three earthquakes, and 650,000 of those were from a single earthquake in the city of Tangshan in 1976. Earthquake destruction is not confined to China. In Italy between 1908 and 1976, three earthquakes killed over 155,000 people. In Peru in 1970, a single earthquake killed 70,000 people. Japan has seen its share of disasters, with nearly 200,000 deaths being blamed on thirteen major earthquakes between 1891 and 1978. The 1995 Kobe earthquake in Japan killed nearly 5,500 people, injured 35,000 others, destroyed or badly damaged nearly 180,000 buildings, and caused damage totaling almost US $147 billion. In the United States, over 1,000 deaths have been attributed since 1906 to eight earthquakes, including the Loma Prieta earthquake in 1989 which claimed 68 lives in the San Francisco Bay area and caused over $20 billion in damage. In 1997, earthquakes were the cause of at least 2,980 deaths around the world.
Ironically, the earthquake itself, considered as the independent natural phenomenon of ground vibration, typically does not pose a threat to humans unless it causes major landslides or tidal waves. Rather, an earthquake typically becomes a dangerous force of nature when the ground vibration it creates interacts with manmade structures, causing gross deformation and structural failure thereof. Structural deformation during seismic excitation is due to forced displacement at the foundation, which results in oscillation and associated horizontal inertial loading on the structure. Because most structures are basically designed for gravitational loading, as opposed to earthquake-induced horizontally directed loading, an earthquake becomes a catastrophic event when structural failure occurs due to the inability of structures to withstand the forces caused by seismic excitation.
In the effort to neutralize the danger caused by collapsing structures during an earthquake, structural engineers have, over the past fifty years, made significant advances in the design of structures for resilience to earthquake excitation. As knowledge has accumulated in this field, it has become evident that in order for a structure to avoid collapse, it must be designed to absorb and dissipate the kinetic energy imparted to it by the earthquake. Modem earthquake-resistant design has basically followed three courses: 1) the design of structures with members able to passively dissipate significant amounts of energy through stable inelastic deformation, while sustaining limited amounts of damage; 2) the use of special energy-dissipating devices for limiting the degree of damage sustained by the structure; and 3) seismic isolation of structures in an attempt to control the amount of energy imparted to them by an earthquake. The advances made in these three areas are implemented not only in new constructions, but also in retrofitting of existing structures.
The oscillation and deformation of a building or other structure due to seismic excitation is a physical process during which kinetic energy is imparted to the structure in the form of elastic deformation. This energy alternates continuously from kinetic to potential (strain) energy during successive phases of oscillation of the structure, until it is ultimately dissipated as heat energy through the procedure of viscous and hysteretic damping. Thus, one of the main problems in designing an earthquake-resistant structure is to provide a structural system able to dissipate this kinetic energy through successive deformation cycles without exceeding certain damage limits. In other words, the building or structure must be able to translate large quantities of kinetic energy into deformations in the plastic range of the construction material. To accomplish this, structures are designed to passively resist earthquake damage through a combination of strength and deformability. The intent of this design approach is for a structure to behave elastically for low-intensity earthquakes, suffering no structural damage, to suffer some repairable damage from medium-intensity earthquakes, and to withstand high-intensity earthquakes without collapsing but suffering significant plastic deformations in critical regions of the structural elements. To achieve this, it is known to provide moment resisting frames, shear walls, concentric and eccentric braces, or a combination of these to increase lateral strength and avoid excessive floor displacement (interstory drift). Under high-intensity earthquakes, the shear walls are permitted to crack and yield, concentric braces are permitted to buckle, and eccentric brace shear links are designed to yield so as to reduce inertial forces during earthquake shaking. Seismically induced damage under moderate and high-intensity earthquakes is intended to occur in specially detailed critical regions of lateral force resisting systems, e.g. in the beams near the beam-column joints. Although this design philosophy gives structures improved ability to avoid collapse, it is untenable to some structural designers charged with designing hospitals, fire departments, and other critical facilities which must remain in operation following a strong earthquake.
The second design course mentioned above, namely use of special energy-dissipating devices, has involved four main groups of devices: friction devices which dissipate energy by way of metal to metal slippage contact, metallic damping devices which exploit reliable yielding properties of mild steel to go through numerous stable inelastic cycles, viscoelastic dampers made of bonded viscoelastic layers (acrylic polymers), and viscous fluid dampers which operate under principles of fluid flow through orifices.
The third conventional approach to the seismic design of structures, that is the base isolation approach, is based on the premise that it is feasible to "uncouple" a structure from the ground and thereby protect it from the damaging effects of earthquake motions. In dynamic terms, the goal is to lengthen the period of vibration of the total system beyond the predominant ground periods, thereby reducing the forced response in the structure. To achieve this result, flexible mounting of the structure is provided by the use of special bearing seats, such as elastomeric/rubber bearings or PTFE/friction sliding bearings, which are installed at the base of the structure between the foundation and the structure. However, the elastomers are subject to aging and sliding surfaces subject to wear, and may not be in a condition to react as intended by the designer at the time of an earthquake.